The present invention relates to a liquid crystal display device and a method for fabricating the same. More particularly, the present invention relates to a liquid crystal display device where liquid crystal molecules are aligned axially symmetrically in liquid crystal regions separated from one another by wall structures.
Conventionally, twisted nematic (TN) and super-twisted nematic (STN) liquid crystal display devices (LCDS) are used as display devices utilizing the electrooptic effect. In order to widen the viewing angle of these LCDS, various techniques are now under vigorous development.
As one of the viewing angle widening techniques proposed so far, Japanese Laid-Open Patent Publication No. 6-301015 and No. 7-120728 disclose a so-called axially symmetrically aligned microcell (ASM) mode LCD where liquid crystal molecules are aligned axially symmetrically in respective liquid crystal regions separated from one another by polymer walls. Each liquid crystal region substantially surrounded by the polymer walls is typically formed for each pixel. Since the liquid crystal molecules are aligned axially symmetrically, such an ASM mode LCD provides a wide viewing angle characteristic where change in a contrast ratio is small whichever direction an observer views the LCD.
The ASM mode LCDs disclosed in the above publications are fabricated by subjecting a mixture of a polymeric material and a liquid crystal material to polymerization induced phase separation.
A method for fabricating a conventional ASM mode LCD will be described with reference to FIG. 9. First, in step (a) shown in FIG. 9, a glass substrate 908 having a color filter and an electrode formed on one surface is prepared. The color filter and the electrode formed on the glass substrate 908 are not shown in FIG. 9 for simplification. Formation of the color filter is described hereinafter.
In step (b), polymer walls 917 for axially symmetrical alignment of liquid crystal molecules are formed in a lattice shape, for example, on the surface of the glass substrate 908 on which the color filter and the electrode are formed. Specifically, a photosensitive resin material is spin-coated on the glass substrate 908, exposed to light via a photomask having a predetermined pattern, and developed to form lattice-shaped polymer walls. The photosensitive resin material may be negative or positive. A non-photosensitive resin material may also be used although a step of forming a resist film is additionally required.
In step (c), column protrusions 920 are formed on the top faces of the resultant polymer walls 917 by dispersive patterning. The column protrusions 920 are obtained by exposing to light and developing a photosensitive resin material as in the polymer walls.
In step (d), the surface of the glass substrate 908 with the polymer walls 917 and the column protrusions 920 formed thereon is coated with a vertical alignment material 921 such as polyimide. Meanwhile, in steps (e) and (f), the surface of a counter substrate 902 on which an electrode is formed is also coated with the vertical alignment material 921.
In step (g), the resultant two substrates are bonded together with the surfaces thereof on which the electrode is formed facing each other, to form a liquid crystal cell. The gap between the two substrates (cell gap, i.e., the thickness of a liquid crystal layer) is defined by the sum of the height of the polymer walls 917 and the height of the column protrusions 920.
In step (h), a liquid crystal material is injected into the resultant cell gap by a vacuum injection method or the like. Finally, in step (i), a voltage is applied between a pair of opposing electrodes to align liquid crystal molecules axially symmetrically in each liquid crystal region 916. That is, liquid crystal molecules in the liquid crystal region 916 defined by the polymer walls 917 are aligned axially symmetrically with respect to an axis 918 (vertical to the substrates) shown by the dashed line in FIG. 9.
FIG. 10 shows a cross-sectional structure of a conventional color filter. The color filter includes colored resin sections of red (R), green (G), and blue (B) corresponding to respective pixels and a black matrix (BM) film for light-shading the gaps between the colored resin sections, which are formed on a glass substrate. The colored resin sections and the BM film are covered with an overcoat (OC) layer made of an acrylic resin or an epoxy resin having a thickness of about 0.5 to 2.0 μm for improving the smoothness and the like. The OC layer is then covered with an indium tin oxide (ITO) film as a transparent signal electrode. The BM film is generally made of a metal chromium film having a thickness of about 100 to 150 nm. The colored resin sections are made of resin materials colored with a dye or a pigment and generally have a thickness of about 1 to 3 μm.
The color filter is formed by patterning photosensitive colored resin layers formed on the substrate by photolithography. For example, R, G, and B photosensitive resin materials are individually subjected to the process of layer formation, light exposure, and development (the process is done total three times), to form the R, G, and B color filter sections. Each photosensitive colored resin layer can be formed by applying a liquid photosensitive colored resin material (obtained by diluting the material with a solvent) to the substrate by spin coating or the like, by transferring a dry film of a photosensitive colored resin material, or other methods. Using the thus-formed color filter, the ASM mode color LCD described above having a wide viewing angle characteristic is obtained.
However, the inventor of the present invention has found that the conventional ASM mode LCD has the following problem.
FIGS. 11A and 11B are plan views of a conventional ASM mode LCD (viewed from the direction normal to the display plane): FIG. 11A schematically illustrates a corner of a liquid crystal region and the orientation state of liquid crystal molecules; and FIG. 11B schematically illustrates the arrangement of a plurality of liquid crystal regions. As shown in FIGS. 11A and 11B, in the conventional ASM mode LCD, polymer walls 917 (wall structures) for aligning liquid crystal molecules axially symmetrically are formed in a lattice shape defining rectangular liquid crystal regions 916. It has been found from examinations performed by the inventor that the right-angle corners of the rectangular liquid crystal regions 916 possess a degree of steepness that is not negligible for the liquid crystal molecules in consideration of the size of the molecules. Therefore, in a corner 917a of the liquid crystal region 916, the alignment direction of the liquid crystal molecules with respect to the surface of polymer walls 917 is irregular as shown in FIG. 11A, losing continuity in the alignment of the liquid crystal molecules. This may cause a variation in the viewing angle characteristic of the liquid crystal display device and result in rough display.
In order to avoid the disturbance in the alignment of the liquid crystal molecules in the periphery of each liquid crystal region (near the polymer walls) from influencing the display, a black matrix is conventionally formed to shade light transmitted through the periphery (including the corners) of the liquid crystal region. This formation of the black matrix minimizes roughness of display, but lowers the aperture ratio. That is, the brightness is sacrificed.